1. Technical Field
The present invention relates to a SAW device, a SAW oscillator, and an electronic apparatus.
2. Related Art
A SAW device (surface acoustic wave device) is a circuit element that converts an electric signal into a surface wave for signal processing and widely used as a filter, a resonator, and other components. A known configuration of such a SAW device includes a SAW chip formed of a piezoelectric substrate made of quartz or any other piezoelectric material and an IDT electrode (comb electrode) provided on the piezoelectric substrate and a base substrate to which the SAW chip is fixed with an adhesive (see JP-A-2005-136938, for example).
The SAW device described in JP-A-2005-136938 includes a SAW chip having an IDT electrode provided on a quartz substrate and a base substrate that supports the SAW chip with an adhesive therebetween. In JP-A-2005-136938, the SAW chip has a mounting portion that is located in one end portion of the SAW chip but does not overlap with the IDT electrode, and the mounting portion is bonded to the base substrate with an adhesive therebetween, whereby the SAW chip forms a cantilever supported by the base substrate. It is known that supporting the SAW chip as a cantilever as described above improves an aging characteristic of the SAW chip (reduces the amount of time-course variation in oscillating frequency).
Further, in the SAW device described in JP-A-2005-136938, pads formed on the SAW chip are electrically connected to pads formed on the base substrate with wires (bonding wires).
In the thus configured SAW device, the SAW chip needs to be firmly bonded to the base substrate. The firm bonding suppresses unnecessary vibration of the SAW chip produced when the wires (bonding wires) are connected to the SAW chip in an ultrasonic bonding process, whereby the wires can be more firmly bonded to the SAW chip.
The SAW device is further preferably so configured that the SAW chip is parallel to the base substrate in order to prevent the SAW chip from being broken or fractured. The parallel configuration prevents the free end of the SAW chip from coming into contact with the base substrate or other components, effectively preventing the SAW chip from being broken or fractured. The parallel configuration further allows ultrasonic vibration to be efficiently applied to the SAW chip when the wires are connected to the SAW chip in the ultrasonic bonding process, whereby the wires can be more firmly bonded to the SAW chip.
To meet the requirements described above, it is conceivable to increase the area where an adhesive is applied onto the SAW chip. In this case, however, the state in which the base substrate supports the SAW chip changes as the adhesive area increases from “cantilever support” to “entire-area support.” The change in the state degrades the aging characteristic of the SAW chip.
To address the problem, in the SAW device descried in JP-A-2005-136938, the SAW chip has a relatively large mounting portion formed in one end portion thereof, and the mounting portion is fixed to the base substrate with an adhesive to achieve a state in which the SAW chip is cantilever-supported by and firmly bonded to the base substrate. In the SAW device described in JP-A-2005-136938, relatively high-level aging characteristic, bonding strength, and parallelism are therefore achieved.
In consideration of characteristics and precision of a SAW device that are required in recent years and in the future, however, the SAW device described in JP-A-2005-136938 is insufficient in terms of the requirements described above (bonding strength and parallelism, in particular) and hence required to be improved.